Does The Location of An Idler Pulley/Bearing Affect The Usable Life of A Belt?

So the thought kinda hit me and I think the answer may be yes I wanna get opinions on this.

So for those who arent aware, idler pulleys/bearings force the belt into a path that engages more teeth, making torque transfer better and helps prevent belt slippage from loose belts.

So my question is, when installing an idler pulley, should you mount it on the top or bottom portion of the belt? Both will increase the number of engaged teeth on the motor pulley but, and here’s where I dont quite know how to quantify the feeling, that placing on the top section of the belt, the portion that is under tension, would reduce the life of the belt from excess wear.

So my reasoning is on a idler pulley free set up, the top portion of the belt in under tension but is straight, meaning the tension balances itself out in regards to the set up. (two forces in equal and opposite directions). But when you introduce an idler pulley on the top portion, you get an imbalance that the pulley must make up for. You have two equal forces at an angle to each other and the balancing force is applies to the belt by the idler pulley. The point should balance out to a zero sum in a free body diagram of that point on the belt. This means there’s more wear on the backside of the belt, which is going to happen due to the contact but I think that can be minimized.

I think if you place the idler pulley on the bottom portion of the motor mount and engage the lower portion of the belt that is not under tension, and is also not under compression because belts, chains and ropes cannot be put under compression, they fold. I think you can significantly reduce the wear and tear on the exterior portion of the belt exploiting this fact.

Slight disclaimer, the forces experience by the belt are actually not equal and but are opposite. There is a net force in the direction of travel which is why the belt rotates under the motors applied toque but I’m ignoring that for the purposes of making this easier to think about conceptually.

No Idler Pulley Where both forces are equal and opposite on one axis, no external wear resulting.

Top Side Idler Pulley Where A+A+B = 0 in a vector summation. bottom portion is still not under tension.

Bottom Side Idler Pulley Where a+b+c=0 but the magnitude of the tension is significantly less - just enough to change the path of the belt but no more - than that of the top portion of the belt, which is the driving portion of the belt. Because belts cannot be under compression.

This of course doesnt account for braking in which the tension switched from top to bottom but assuming we spend most of the time cruising rather than breaking I think the theory still holds. You can of course set the belt tension so high that in these examples, the lower portion of the belt is already under tension and instead of not being under tension during use, it simply lowers the magnitude of the tension on the bottom side but even then it still proves the point.


Interesting theory. Makes sense to me, but I’m not smart enough to know if it would translate to a difference in the real world. You could run a dual motor experiment with an upper idler pulley on one side and a lower idler pulley on the other and ride hard till one breaks.

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its an interesting question top or bottom… I have done both and haven’t really seen a plus or minus to either way, they both work out basically the same.

one thing is certain. wherever you put the idler it definitely performs better than none.


From our own testing you get better power transmission if you keep the idler to the slack side of the belt drive. Adding an idler actually increases the life of belts and allows you to run looser belts without slippage. Having more teeth in mesh also reduces wear on the motor pulley by reducing the load on each individual tooth.


You’re saying that adding the idler to the slack side increases the life of the belt when compared to an idler free set up, that I was not expecting.

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this is also true. there is no question in my mind that having an idler is much better than not having one.


Would there be any advantage to using an idler pulley on each side of the belt?

if you want to keep the centre distance short but want to have a massive gear down, like for an off road board. sure. but if you don’t need to why over complicate it.

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Ditto. I think it would be worse than just using a shorter belt length and just a single idler on the bottom.

Great post @JuniorPotato93!

It was honestly inspired by from scrolling through pictures on here so but I think it’s a topic that may be more intuitive for people to want to ask, I just happened to do it.

You have diagrams and shit…

A++ for effort and execution

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LOL yeaaahh. The diagrams are pretty bad I will admit that whole heartedly. I actually almost uploaded this without the diagrams but felt they did a little more good than bad.

Sorry for the ignorant question but, which is the slack side?

bottom side if inward mounted, top side if outward mounted. Basically, it’s the side the motor doesn’t tug on.


Too funny that this discussion came up. I’m currently adding electric power to my favorite longboard, and since I don’t like the way existing mounts tension the belt by effectively spacing the motor (they are difficult to adjust well), I had already planned on using an idler pulley (I was also considering using fewer teeth on the motor pulley, so being able to force the engagement of additional teeth was another plus), AND I was going to put it on the slack/bottom side. That just seemed logically to be slightly better (for all the reasons discussed here). Also, being on the bottom would allow for easy tension adjustments because the idler pulley would be within easy reach once the board was flipped over.

Here’s a link to the idler pulley I ordered:

well after some weeks of testing I can confirm that having the idler pulley on the “bottom” or looser part of the belt has made a huge difference. there is no belt slip, the board is faster and more efficient.


If you use car, motorcycle and other machinery manufacturers as a model, idlers and tensioners, on timing belt drive systems, seem to be always on the slack side of the belt so to speak. Cam belts are a particularly common example. I’m sure there will be a few exceptions but this would seem to be the best way. I wouldn’t want to have to explain the reasoning but I reckon putting a tensioner on the tight side of the belt will result in an increase in frictional power losses. Andy.

This is a cool idea. Nice job. Where would I pick up an idler pulley from. I have no clue what size, type, ect… I have enertion motor mounts I’m working with.

Easy to make. Use a 6 or 8mm bolt, a spacer or 2 and a couple of bearings. Drill a proper hole in your mount and tap it to the appropriate size.